1. Field of the Invention
The present invention relates to an antenna for transmitting and receiving analog or digital radio frequency signals in a frequency band of the microwave band or higher, mainly in a frequency band of the millimeter-wave band. More particularly, the present invention relates to an endfire antenna apparatus, efficiently radiating in a direction parallel to a substrate that is provided with a plurality of conductive elements composing the antenna.
2. Description of the Related Art
In recent years, it has been considered to adopt millimeter-wave radio techniques not only to an onboard radar for cars, but also to a wireless LAN (Local Area Network) and a wireless PAN (Personal Area Network). In order to provide a small-sized terminal with a millimeter-wave radio unit, it is essential to reduce the antenna size, i.e., to reduce the thickness of a circuit board including the antenna unit, and to reduce the area of the circuit. Meanwhile, as compared to the case of the microwave band, the propagation loss increases in the case of using the millimeter-wave band, nevertheless, it is difficult to implement a transmission system with high-power output in that case. Thus, as a consequence, an antenna requires high gain characteristics.
As millimeter-wave band antennas for use in onboard radars, high-gain dielectric leaky-wave antennas are known that converts leaky waves on dielectric, transmitted along an interface between the dielectric and air, into radiation components, as disclosed in Patent Documents 1 to 3 and in Non-Patent Document 1. Patent Document 1 discloses a dielectric leaky-wave antenna provided with: a ground plate conductor; a dielectric substrate provided on one side of the ground plate conductor, and forming a transmission path between the ground plate conductor and the dielectric substrate for transmitting an electromagnetic wave along its surface from one end to the other end; loading elements loaded on the dielectric substrate, and for leaking the electromagnetic wave out of the surface of the dielectric substrate; and a feed unit for supplying the electromagnetic wave at the one end of the transmission path formed between the ground plate conductor and the dielectric substrate. The dielectric leaky-wave antenna is characterized in that a dielectric layer with a permittivity lower than that of the dielectric substrate is provided between the ground plate conductor and the dielectric substrate. The loading elements are a plurality of metal strips placed in parallel to each other at intervals of a certain distance “d”, and to be orthogonal to a transmission direction of the electromagnetic wave in the transmission path. The loading elements are formed on the front side of the dielectric substrate, which opposite to the side of the dielectric layer. Furthermore, the loading elements convert a part of the electromagnetic wave propagating through the dielectric substrate, into leaky waves on the dielectric.
According to Patent Document 1, in order to leak the leaky waves on the dielectric in a direction of angle φn with respect to an axis orthogonal to the dielectric substrate, an adjacent distance “d” of the loading elements must satisfy the following equation:
                                                                                                   sin                  ⁡                                      (                                          ϕ                      ⁢                                                                                          ⁢                      n                                        )                                                  =                                ⁢                                                      (                                                                  β                        /                        k                                            ⁢                                                                                          ⁢                      0                                        )                                    +                                      n                    ⁡                                          (                                              λ                        ⁢                                                                                                  ⁢                                                  0                          /                          d                                                                    )                                                                                                                                                                =                                    ⁢                                                            (                                              λ                        ⁢                                                                                                  ⁢                                                  0                          /                          λ                                                ⁢                                                                                                  ⁢                        g                                            )                                        +                                          n                      ⁡                                              (                                                  λ                          ⁢                                                                                                          ⁢                                                      0                            /                            d                                                                          )                                                                                            ,                                                                        (        1        )            where “λ0” denotes a free-space wavelength, “λg” denotes a guide wavelength inside the dielectric transmission path, “β” denotes a propagation constant of the dielectric transmission path, “k0” denotes a free space propagation constant, and “n” denotes an integer. When discussing radiation components parallel to the dielectric substrate, which is an object of this application and Patent Document 1, the angle “φn” is 90 degrees. When selecting the adjacent distance “d” of the loading elements by the condition of an endfire radiation including only a radiation wave of n=−1, the adjacent distance “d” of the loading elements satisfies the following equation:
                                                                                 d                =                                ⁢                                  λ                  ⁢                                                                          ⁢                                      0                    /                                          [                                                                        (                                                      λ                            ⁢                                                                                                                  ⁢                                                          0                              /                              λ                                                        ⁢                                                                                                                  ⁢                            g                                                    )                                                -                        1                                            ]                                                                                                                                                                ≅                                    ⁢                                      λ                    ⁢                                                                                  ⁢                                          0                      /                                              [                                                                                                            ɛ                              r                                                                                -                          1                                                ]                                                                                            ,                                                                        (        2        )            where “∈r” denotes a relative permittivity of the dielectric substrate.
Non-Patent Document 1 discloses an exemplary design of a dielectric leaky-wave antenna that achieves a gain of about 30 dBi with an efficiency of about 60 to 70%, using the technique of Patent Document 1. According to FIG. 5 and Table 3 in Non-Patent Document 1, since a dielectric substrate (aperture) has dimensions of 60×60 mm, and metal strips (loading elements) are placed at intervals of a distance d=1.7 mm, it can be seen that the dielectric leaky-wave antenna of Non-Patent Document 1 has 30 or more metal strips placed periodically.
Additionally, according to Patent Document 1, in order to suppress reflections in the transmission path caused by the loading elements, the dielectric leaky-wave antenna is further provided with another set of metal strips for loading elements (hereinafter, referred to as the second loading elements) so as to make pairs with the respective metal strips for the aforementioned loading elements (hereinafter, referred to as the first loading elements). The metal strips for the second loading elements are placed in parallel to each other at intervals of a adjacent distance “d”, and are formed on the side of the dielectric substrate opposite to the side of the first loading elements (i.e., the side facing to the dielectric layer). Further, the metal strips for the second loading elements are displaced by λg/4 from the metal strips for the first loading elements, along the transmission direction of the transmission path, where λg denotes the guide wavelength inside the transmission path. Each first loading elements and each second loading elements act as a circuit of a pair of the loading elements to cancel the reflections by each other.
Meanwhile, Patent Document 2 discloses a dielectric leaky-wave antenna is provided with a plurality of leaking metal strips in parallel to each other at intervals of a certain distance, on a front side of a dielectric substrate. Each of the leaking metal strips is composed of two metal strips parallel to each other and spaced apart by about λg/4. The leaking metal strips act in the same manner as that of the loading elements in Patent Document 1. Patent Document 3 discloses an example provided with, in addition to the metal strips for the first and second loading elements of Patent Document 1, outgoing metal strips on another wiring layer for rotating the polarization of an electromagnetic wave to be radiated. According to the purpose of the outgoing metal strips, they are oriented at a different angle than that of the metal strips for the first and second loading elements.    (1) Patent Document 1: Japanese Patent Laid-Open Publication No. 2001-320229,    (2) Patent Document 2: Japanese Patent Laid-Open Publication No. 2003-158420,    (3) Patent Document 3: Japanese Patent Laid-Open Publication No. 2002-237716, and    (4) Non-Patent Document 1: T. Teshirogi, et al., “High-efficiency, dielectric slab leaky-wave antennas”, IEICE Transactions on Communications, Institute of Electronics, Information and communication Engineers (IEICE), Vol. E84-B, No. 9, pp. 2387-2394, September 2001.
As is apparent from Patent Documents 1 to 3, when the length of a dielectric substrate for generating spatial harmonics and for leaking leaky waves on the dielectric out of its surface (i.e., the length of a region where metal strips for loading elements are placed) cannot be considered to be sufficiently longer than the free-space wavelength λ0, the conventional design principles of dielectric leaky-wave antennas cannot be adopted, and thus, it becomes hard to achieve high gain characteristics. Specifically, if determining the adjacent distance “d” of the loading elements so as to satisfy the equation (2) under the condition of short length of the dielectric substrate, then only a small number of loading elements or pairs of loading elements can be placed.
According to Patent Document 1, the dielectric leaky-wave antenna is provided with the loading elements on the front and back sides of the dielectric substrate, at intervals of a distance corresponding to ¼ of the guide wavelength λg inside the transmission path. According to Patent Document 2, the dielectric leaky-wave antenna is provided with the additional loading elements on the front side of the dielectric substrate, spaced apart by the distance corresponding to λg/4. However, these loading elements are not added for the purpose of increasing gain, as clearly mentioned in Patent Documents 1 and 2. According to Patent Document 3, although a metal strip structure on a third layer is newly introduced, this structure is not intended to increase gain, either.